Aqueous dispersions of dyes or pigments have heretofore been widely used in connection with various applications, for example, in dope-dyeing of vinylon or viscose, paper-making, aqueous paints, and as colorants for metals such as aluminum and iron, and the like.
In particular, an aqueous dispersion of carbon black is in use in photographic light-sensitive materials mainly for obtaining black color or light barrier properties, as well as in aqueous printing inks, aqueous paints, and cosmetics.
Carbon blacks are grouped into furnace black, channel black, lamp black, thermal black, acetylene black, etc., according to production process, and they are different from each other in surface properties such as surface functional groups and surface area. Surface functional groups include a carboxyl group, a phenolic hydroxy group, a quinone group and a lactone group. These groups have been observed through infrared absorption spectrum, polarograph, or the like (J. V. Hallum et al., J. Phys. Chem., 62, 110 (1958) and M. L. Studbaker et al., Ing. Eng. Chem., 48, 162 (1956)). These functional groups are chemically adsorbed directly on carbon black particles or, in other cases, polycyclic aromatic substances having quinone groups, carboxy groups, or the like adsorbed on or occluded in carbon black particles.
In addition, carbon black generally contains sulfur compounds or the like and adsorbs various substances depending upon the physical conditions of the surface such as surface area and porosity. Therefore, carbon black is a substance fairly active chemically and physically. On the other hand, in preparation of an aqueous dispersion of carbon black, it is of importance to stably disperse carbon black particles in water in order to improve quality. In general, dyes and pigments (hereinafter dyes and pigments being referred to as "pigments") are dispersed in an aqueous system according to the following processes of:
(1) finely pulverizing a pigment through mechanical means such as an atomizer, ball mill, pebble mill, colloid mill, etc.; and PA1 (2) forming primary particles of the pigment using a dispersing agent such as a surface active agent or a phosphate.
A dispersion state of a pigment attained by applying mechanical or physical energy as process (1) is not stable and, in order to obtain a stable dispersion system, dispersion process (2) using a dispersing agent such as a surface active agent is employed. With pigments poorly wettable with water such as carbon black, it is generally difficult to obtain a stable dispersion system without using a dispersing agent.
It is well known to use a carbon black aqueous dispersion in a photographic light-sensitive material system (Research Disclosure, 15162, November 1976). A carbon black aqueous dispersion for use in the photographic light-sensitive material system has so far been prepared in the same manner as an ordinary carbon black dispersion by adding powder type or bead type carbon black to water having dissolved therein a water-soluble surface active agent (for example, salts of polyacrylic acid, alkylnaphthalenesulfonic acid, formaldehyde-condensed naphthalenesulfonic acid, or polystyrenesulfonic acid as described in Research Disclosure, 15162 (November 1976)), coarsely dispersing carbon black using a coarsely dispersing machine such as a high-speed stirring tyep dispersing machine (e.g., a dissolver as described in Japanese patent application (OPI) No. 129136/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")), and finely dispersing the resultant carbon black using a fine dispersing machine such as a sand grinder, homogenizer, colloid mill, or the like to obtain an aqueous dispersion of carbon black particles having a mean particle size of about 0.1 to about 10 .mu.m.
In an instant film of the type as described in Photogr. Sci. Eng., 20, 155 (1976), it is known to use a carbon black aqueous dispersion in a processing solution or in a light-sensitive layer for the purpose of intercepting light. However, the use of the carbon black dispersion prepared according to the above-described conventional dispersion process in this type processing solution has involved the following defects.
That is, in comparison with the case of using a processing solution not containing the carbon black aqueous dispersion (i.e., processing in dark room), deteriorated photographic properties result (maximum density of dye and minimum density), serious deterioration of the processing solution with time, and poor preservability of processed light-sensitive materials.
Analysis of the causes for these defects by the present inventors has led to the following conclusions:
(1) Since carbon black itself is chemically and physically active as described hereinbefore, it mutually acts with the ingredients in the processing solution (for example, developing solution) to gradually deteriorate properties of the solution. This phenomenon becomes more conspicuous as the time of mechanically pulverizing the carbon black becomes longer. This deterioration is caused by chemical activation of the surface of carbon black particles due to application of mechanical energy whereby the surface is flawed.
(2) The surface active agent used for dispersing carbon black particles penetrates into a light-sensitive material upon processing it to adversely affect development of a photographic emulsion or deteriorate film properties of the processed light-sensitive material, thus exerting unfavorable influences on preservability of the light-sensitive material. This tendency becomes more conspicuous as the amount of the surface active agent is increased for improving dispersibility of carbon black.
With conventional carbon black aqueous dispersions, surface active agents must be used in large amounts for improving dispersibility of carbon black. The use of the surface active agent in a large amount causes foaming to decrease workability, forms pinholes which causes incomplete interception of light when the resulting dispersion is used in a processing solution or in a light-sensitive layer for intercepting light, and causes uneven coating.
On the other hand, an aqueous dispersion of a dye or a pigment other than carbon black is prepared by dispersing a varying organic dye or pigment or inorganic pigment in an aqueous medium using a varying dispersing agent. However, serious foaming occurs upon dispersing the dye or pigment due to the action of the surface active agent, which deteriorates workability.
A further general problem is aggregation and sedimentation of dye or pigment particles during storage, which leads to deterioration of a coloring effect on materials to be colored.